This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system wherein a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing the dark-light variations in the document being scanned. These information-bearing signals are then transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the original document.
In several commercially available facsimile systems, the information-bearing signals which are transmitted over the communications network are FM (frequency modulated) signals. In general, these signals lie in an FM bandwidth of 1500 to 2400 Hz. which represents the audio range which is transmittable over ordinary telephone lines. Where this frequency range is utilized, the 1500 Hz. signals usually represent a white level, the 2400 Hz. signals represent the black level and signals in the frequency range between 1500 and 2400 Hz. represent varying degrees of gray. Copending U.S. application Ser. No. 417,797, filed Nov. 21, 1973, now U.S. Pat. No. 3,911,207 discloses a specific technique for demodulating the FM signals using a single-shot multivibrator which generates trigger signals having a frequency proportional to the frequency of the FM signals received by the facsimile receiver. Such a demodulation technique is desirable in that it is low cost and reliable in the absence of noise. In the presence of noise or input frequency components outside the 1500-2400 Hz. band of interest, the trigger signals may trigger the single-shot multivibrator at a frequency proportional to the noise or undesired frequency components, as well as the information-bearing FM signals representing the dark-light variations in the document being transmitted. In addition to producing poor copy at the receiver, the presence of noise or input frequency components outside the band of interest can actually shut the receiver down where control circuitry responsive to a poor signal transmission is utilized.
This control circuitry, which is disclosed in the aforesaid copending application, is designed to shut the receiver down when the received signal frequencies fall below 1500 Hz. for a predetermined length of time thereby indicating either a loss of signal or insufficient components of frequencies in the desired band of interest from the transmitter. However, it is possible to erroneously activate this control circuitry by the output of the single-shot multivibrator since the single-shot multivibrator can erroneously be triggered by a distorted input signal with frequency components below 1500 Hz. and the signal at the output of the multivibrator appears the same as it would if the signals being received had a frequency less than 1500 Hz.
Copending application Ser. No. 333,616, filed Feb. 20, 1973, now U.S. Pat. No. 3,872,239 discloses a receiver for demodulating signals representing dark-light variations on a document. The receiver also includes a stylus drive circuit having a brightness control potentiometer connected in the base circuit of a pair of transistors. There is no suggestion as to how contrast control might be achieved alone or in combination with brightness control.